The present invention relates to a single crystal manufacturing apparatus for manufacturing, for example, a silicon single crystal used as a semiconductor material and, more particularly, to a single crystal manufacturing apparatus having a magnet unit for applying a magnetic field to a single crystal raw material melting liquid.
FIG. 1 shows an example of the structure of a conventional single crystal pulling apparatus by means of the Czochralski method (a CZ method). A crucible 2 in which a single crystal raw material melting liquid (hereafter abbreviated as "a melting liquid") 1 is filled, is heated by a heater 3, and the single crystal raw material is always maintained in a melted state. When a seed crystal 4 is inserted into the melting liquid and the crystal 4 is then pulled by a pull driving mechanism 5 at a predetermined speed, a crystal is grown on a solid-liquid surface boundary layer 6 disposed in a liquid surface range of the melting liquid 4, and a single crystal 7 is produced. In this case, a fluid motion of the melting liquid 1 induced due to the heating of the heater 3, i.e., a thermal convection 8 occurs.
The reason that the thermal convection 8 occurs will be described as follows: The thermal convection 8 occurs in general when a balance between a buoyancy of a fluid caused by a thermal expansion and a viscous force of the fluid is broken down. The no dimensional quantity for representing the balance relationship between the buoyancy and the viscous force is represented by the following Grashof number N.sub.Gr : EQU N.sub.Gr =g.multidot..alpha..multidot..DELTA.T.multidot.R.sub.3 /.nu..sup.3
where
g: gravity acceleration
.alpha.: thermal expansion coefficient of melting liquid
T: radial temperature difference of crucible
R: radius of crucible
.nu.: dynamic viscous coefficient of melting liquid
When the Grashof number N.sub.Gr generally exceeds the critical value determined by the geometrical size of the melting liquid 1 and thermal environmental conditions, a thermal convection 8 occurs in the melting liquid 8. Normally, the thermal convection 8 of the melting liquid 1 becomes a turbulent state when N.sub.Gr &gt;10.sup.5 and becomes a disturbing state when N.sub.Gr &gt;10.sup.9. The Grashof number becomes N.sub.Gr &gt;10.sup.9 under the melting liquid conditions when pulling the single crystal of 3 to 4 inches in diameter carried out at present (according to the equation of the N.sub.Gr). The melting liquid 1 becomes a disturbing state, and the liquid surface of the melting liquid 1, i.e., the solid-liquid surface boundary layer 6, becomes wavy.
When the thermal convection 8 of the above-described disturbing state occurs, the temperature variations in the melting liquid 1, and particularly in the solid-liquid surface boundary layer 6, becomes vigorous, the positional and timing variations of the thickness of the solid-liquid surface boundary layer 6 become vigorous, and the microminiature remelting of the crystal during the growth becomes remarkable. Thus, dislocation-loop, laminated-layer defects occur in the grown single crystal 7. Further, the defective part irregularly occurs with respect to the pulling direction of the single crystal due to the irregular variations in the solid-liquid surface boundary layer 6.
Further, impurities 9 melted in the melting liquid 1 are conveyed to the thermal convection 8 from the inner surface of the crucible 2 due to the chemical change between the melting liquid 1 and the crucible 2 on the inner surface of the crucible 2 where the melting liquid 1 of high temperature (e.g., approx. 1,500.degree. C.) is contacted, and dispersed over the entire interior of the melting liquid 1. The impurities 9 become nuclei, a dislocation loop, defects, and/or grown fringes occur in the single crystal 7, and the quality of the single crystal 7 is deteriorated. Therefore, when a wafer of an LSI(Large Scale Integration; large scale integrated circuit) is manufactured by such a single crystal 7, the wafer is impossible to use since the wafer including the defective part is accordingly deteriorated in the electric characteristics, and the yield of the wafer becomes wrong.
The diameter of the single crystal 7 will be remarkably increased in the future, but the larger the diameter of the crucible 2 increases, the larger the Grashof number increases, and the more the thermal convection 8 of the melting liquid 1 becomes vigorous, as understood from the above-described equation of the Grashof number. Thus, the quality of the single crystal 7 is directed toward deterioration. To this end, a method of applying a D.C. magnetic field to the melting liquid 1 so as to suppress the thermal convection 8 and pull a single crystal under the growing conditions near the thermal and chemical equilibrium state has been proposed.
FIG. 2A shows an example of a conventional single crystal manufacturing apparatus using a method of applying a magnetic field. In FIG. 2A, the same reference numerals as in FIG. 1 denote the same parts in FIG. 2A, and the description thereof will be omitted. In FIG. 2A, a magnet 10 which is composed of a copper coil for applying a uniform magnetic field in a direction designated by 11 being perpendicular to the pulling direction of a single crystal in a melting liquid 1 is disposed on the outer periphery of a crucible 2.
The melting liquid 1 of the single crystal 7 is in general a conductor having an electrical conductivity .mu.. Thus, when a fluid having the electrical conductivity is moved due to the thermal convection 8, the fluid moved in a direction not parallel to the direction 11 of applying a magnetic field is affected by the influence of a magnetic reluctance force by means of Lenz's law. Therefore, the motion of the thermal convection 8 is prevented.
Generally, the magnetic reluctance force when a magnetic field is applied, i.e., magnetic viscosity coefficient .nu..sub.eff, is: EQU .nu..sub.eff =(.mu.HD).sup.2 .sigma./.rho.
where
.mu.: magnet1c permeability of the melting liquid
H: intensity of the magnetic field
D: diameter of the crucible
.sigma.: electrical conductivity of the melting liquid
.rho.: density of the melting liquid
When the intensity of the magnetic field increases, the magnetic viscosity coefficient .nu..sub.eff increases, and the .nu. in the equation of the above-described Grashof number increases. Thus, the Grashof number abruptly decreases, and the Grashof number can be reduced to be smaller than the critical value due to a certain intensity of the magnetic field. In this manner, the thermal convection of the melting liquid 1 can be completely suppressed.
Since the thermal convection can be suppressed by applying the magnetic field as described above, the aforementioned impurities in the single crystal 7, the dislocation loop, the defects and the grown fringes can be eliminated, and the single crystal 7 having uniform quality in the pulling direction of the single crystal can be obtained, thereby improving the quality and yield of the single crystal 7.
However, the conventional single crystal pulling apparatus having a normal conductive electromagnet composed of a copper coil as shown in FIG. 2A has the following drawbacks. In a large-sized single crystal pulling apparatus used for growing a single crystal having four inches or longer of the size, a housing 12 for containing a crucible 2 and a heater 3 becomes large with, for example, 900 mm or larger of a diameter. When a predetermined intensity B.sub.1 of magnetic field (e.g., 2,000 gausses or higher) is intended to be produced in a solid-liquid surface boundary layer 6 by a magnet 10 mounted on the outer periphery of the housing 12 so as to suppress the thermal convection of the melting liquid 1, a very large coil for producing a large ampere-turn is necessary from the result of a calculation by Biot-Savart's law. For example, 10.sup.6 amperes-turn or higher is necessary.
In order to obtain such a large ampere-turn by the normal conductive electromagnet, the diameter of the coil becomes very large, so that the inner diameter of the coil normally becomes sufficiently larger than the height of the crucible 2, with the result that the cucible 2 is entirely covered by the coil. Thus, the distribution 13 of the magnetic field in the crucible 2 becomes substantially uniform with respect to the height direction of the crucible 2 as shown in FIG. 2B. Thus, the relationship between the intensity B.sub.1 of the magnetic field in the solid-liquid surface boundary 6 and the intensity B.sub.2 of the magnetic field in the lower portion of the crucible 2 normally becomes .vertline.B.sub.1 -B.sub.2 /B.sub.1 .vertline.&lt;5%.
Therefore, the Grashof number distribution of the melting liquid corresponding to the distribution 13 of the magnetic field becomes as designated by a curve 14, and all become equal to or lower than the critical Grashof number N.sub.Gc, where N.sub.G1 and N.sub.G2 represent the Grashof numbers of the melting liquids in the solid-liquid surface boundary layer 6 and the bottom of the crucible 2. Consequently, the thermal convection of the melting liquid 1 in the crucible 2 is suppressed anywhere, and the melting liquid 1 becomes completely steady. In this state, the transfer path of the heat due to the transfer of the thermal convection is eliminated, and the supply of the heat from the heater 3 to the melting liquid 1 is only by the thermal conduction of the melting liquid.
When the size of the single crystal is small such as 2 to 3 inches, the crucible 2 becomes small such as 4 to 6 inches. Even when the melting liquid 1 is completely steady due to the application of the magnetic field, the heat supplied from the heater 3 is sufficiently transferred to the solid-liquid boundary layer 6 due to the thermal conduction of the melting liquid 1, with the result that there is almost no temperature difference between the solid-liquid surface boundary layer 6 and the periphery of the crucible 2. More particularly, the temperature difference is normally suppressed to ten and several degrees in Centigrade or less.
On the other hand, in the large-sized single crystal pulling apparatus having four inches or larger of the single crystal size, the diameter of the crucible 2 increases to 10 to 14 inches. Thus, the heat of the heater 3 is not sufficiently transferred to the solid-liquid surface boundary layer 6 disposed at the center of the crucible 2 only by the thermal conduction. Therefore, a large temperature difference occurs between the solid-liquid surface boundary layer 6 and the periphery of the crucible 2. More specifically, the difference normally becomes several tens of degrees in Centigrade.
In order to effectively grow the single crystal 7 in the solid-liquid surface boundary layer 6, a sufficient temperature higher than a predetermined value is required. For example, in case of a silicon single crystal, 1,400.degree. C. or higher is required. Therefore, it is necessary to increase the power of the heater and to apply the temperature of the predetermined value to the solid-liquid surface boundary layer 6 by overcoming the temperature gradient thereof.
Further, when the temperature gradient is large, a considerable temperature gradient occurs even in the solid-liquid surface boundary layer 6 as the size of the single crystal is large. In order to grow a uniform single crystal 7, the temperature uniformity in the growing range is also required. Consequently, it is not preferable in the growth of the single crystal that there is such a large temperature gradient. When the temperature difference between the center of the crucible 2 and the periphery thereof is excessively large, the thermal stress acting on the crucible 2 becomes excessive, with the result that the crucible 2 is readily cracked.